The Laigin, modern spelling Laighin (Irish pronunciation: [ˈl̪ˠaːjɪnʲ]), were a population group of early Ireland. They gave their name to the province of Leinster, which in the medieval era was known in Irish as Cóiced Laigen, meaning "province of the Leinstermen". Their territory, located in south-east Ireland, is thought to have once extended from the River Shannon to the River Boyne.
Laigin is a plural noun, indicating an ethnonym rather than a geographic term; however the Irish system of naming territories meant that an area tended to be named after an apical ancestor-figure even when the ruling dynasty had no links to that figure. The origin of their name is uncertain; however, it is traditionally assumed to derive from the Irish word láigen, meaning 'a spear'. Early texts use names Láigen and Gáileoin interchangeably.
The Laigin are claimed as being descended from Labraid Loingsech. Modern historians suggest, on the basis of Irish traditions and related place names, that the Laigin were a group of invaders from Gaul or Britain, who arrived no later than the 6th century BC, and were later incorporated into the medieval genealogical scheme which made all the ruling groups of early Ireland descend from Míl Espáine. Placenames also suggest they once had a presence in north Munster and in Connacht.
Archaic poems found in medieval genealogical texts distinguish three groups making up the Laigin: the Laigin proper, the Gáilióin, and the Fir Domnann. The latter are suggested to be related to the British Dumnonii.
In the legendary tales of the Ulster Cycle, the king of the Connachta, Ailill mac Máta, is said to belong to the Laigin. This is thought by Byrne (2001) to be related to a possible early domination of the province of Connacht by peoples related to the Laigin, the Fir Domnann and the Gamanrad.